Function qsa_gather 

pub fn qsa_gather(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    k_cache: DevicePtr,
    v_cache: DevicePtr,
    block_table: DevicePtr,
    sel: DevicePtr,
    k_out: DevicePtr,
    v_out: DevicePtr,
    n_sel: u32,
    block_size: u32,
    nkv: u32,
    hd: u32,
    stream: u64,
) -> Result<()>

Pack the selected tokens’ K/V rows into contiguous NHD scratch.